The low frequency dielectric constant of aerosolized bacterial cells will be measured on a single particle basis. An initial pilot study is aimed at adapting existing real-time aerosol monitoring and characterizing instrumentation for the rapid dielectric analysis of biological particles. The proposed scheme integrates two proven techniques: laser Doppler velocimetry (LDV) and dielectrophoresis (DEP). Particles subjected to an alternating, nonuniform electric field will experience a force directed toward a high field region and the resulting drift velocity is a quantitative measure of the particle's dielectric constant. A frequency-biased dual-beam LDV system will measure the particle's migration velocity. A flow system will carry the particles through the field gradient and the laser beam intersection region, with a resulting observation time of approximately 0.1 sec. A.C. electric fields in the audio and radiofrequency ranges will be applied so that representative "low frequency" dielectric values are obtained. Dielectrophoretic mobility measurements to within 45% are anticipated. The rapid, automated dielectric measurements proposed here will open up the possibilities for studying (1) sources of polarization within the cell and at the cell surface, (2) variability of dielectric behavior with age, and (3) the feasibility of rapid and automatic separation of cell types based on dielectric properties.